1. Field of the Invention
The present invention is in the field of power converters. The present invention is further in the field of semiconductor switching power converters. The present invention further relates to the field of integrated hysteretic control methods for switching power converters and circuits. The present invention is further in the field of integrated switching power converters. The implementation is not limited to a specific technology, and applies to either the invention as an individual component or to inclusion of the present invention within larger systems which may be combined into larger integrated circuits.
2. Brief Description of Related Art
Modern electronic applications require power management devices that supply power to integrated circuits or more generally to complex loads. In general, power switching converters are becoming more and more important for their compact size, cost and efficiency. The switching power converters comprise isolated and non isolated topologies. The galvanic isolation is generally provided by the utilization of transformers. Although the subject invention is mainly focused on non isolated switching power converters, it refers both to isolated and non isolated power converters.
Modern switching power converters are in general divided in step down power converters also commonly known as “buck converters” and step up power converters commonly known as “boost converters”. This definition stems from the ability of the converter to generate regulated output voltages that are lower or higher than the input voltage regardless of the load applied.
Bidirectional switching power converters are characterized by the ability to operate both as buck converters and as boost converters simply by inverting the flow of power conversion and by utilizing the same switching elements and the same passive components to store energy during the power conversion operation. Typical applications involve the charging and discharging of a battery, hooked to one side of the converter and the use of a connector on the other side of the converter such that the converter can be coupled to a power source to charge the battery or to a load to utilize the energy stored in the battery itself.
A common application could be the charging of a lithium ion battery of a portable device through an Universal Serial Bus (USB) power source external to the portable equipment and the utilization of the battery charge to power an external device connected by means of the USB interface. Nowadays the USB voltage provided is about 5V and the maximum voltage at which the battery can be charged, if lithium ion, is 4.2V-4.3V depending on the specific chemistry and battery manufacturing flow, therefore the charging of the battery takes place by operating the power converter as a buck converter. However when an external device is hooked to the USB interface to be powered up, the voltage to be provided has to be regulated at about 5V, therefore a boost mode operation is requested by the bidirectional power converter to boost up the voltage rail from the battery voltage to 5V.
The main motivation to make use of bidirectional switching power converters is to lower cost and save board space. However the control of a buck converter is quite different from the one for the boost converter because the inherent mode of operation is very dissimilar. The small signal analysis of the boost circuit in CCM (Continuous Conduction Mode) points out the presence of a right half plane zero (RHPZ), which leads to an apparently counter-intuitive decrease of the current in the diode when the load current increases, since the duty cycle increases. This RHPZ can complicate the stability of the loop and generally is dealt with by rolling off the loop gain of the switching voltage regulator at relatively low frequency, making the overall response of the boost converter quite slow.
One example of bidirectional power converter that operates as a buck in one direction and as a boost in the opposite direction swapping input and output depending on the required regulated voltage is described in Dishner (U.S. Pat. No. 4,736,151). Dishner implements the concept with a typical inverting buck boost topology which implies potentially large negative voltages. Another prior art example is described in Canter et al. (U.S. Pat. No. 5,359,280) where a battery is charged or discharged by the converter. A further example is detailed in Linkowsky et al. (U.S. Pat. No. 5,602,464) where a transformer is utilized and the current is sensed in amplitude and sign by a sense resistor in series to one of the winding of the transformer.
Another example of bidirectional buck boost is described in Esser (U.S. Pat. No. 5,734,258) where a four switches buck boost topology is shown. Hack et al. (U.S. Pat. No. 6,894,461) describes a more general approach with multiple control loops. Walter et al. (US Patent Application 2010/0237840) describes one bidirectional power converter that allows the flow of power in both directions depending on a mode selection signal provided to the converter. However this last example, as all the other cited prior art, does not implement the switching power converter with hysteretic control, limiting the use of these power converters only at relatively low operating frequencies In fact Walter et al specifically described the possible control methods clearly omitting the hysteretic or pseudo hysteretic approach due to the intrinsic difficulties in implementing hysteretic control loops, in particular for boost converters.
The limitations described above for boost power converters are not present in buck power converters, in fact faster control methods, hysteretic and pseudo-hysteretic in nature, are becoming increasingly popular for their inherent simplicity and faster control. Generally the boost converters are controlled with PID (proportional-integral-derivative) type of control method. In particular current mode controls are quite common because they include two nested loops: one for the control of the output voltage and one for the control of the output current. However, as mentioned, these types of control methods do not present high bandwidth and require the adoption of large output capacitors to obtain acceptable load transient responses.
High frequency switching power converters show significant advantages over conventional power converters operating at low frequencies, since they allow the use of low value inductors and capacitors reducing significantly the cost and board space of the power management section. Buck converters can successfully be operated at high frequency by using hysteretic and pseudo-hysteretic approaches. Generally the control loop of pseudo-hysteretic converters is relatively simple and the converter's output voltage is summed to a ramp signal to generate a synthetic ripple signal. This signal is then compared to a reference to determine the duty cycle to obtain the desired voltage regulation.
Fast control of boost converters is difficult to obtain in CCM because there is always an intrinsic delay in providing energy to the load since the inductor has to be first charged with current flowing in it. If the load suddenly changes from a low current to a high current load, the boost converter circuit has to spend some time to charge the inductor first and during this time no current/energy is supplied to the load. This phenomenon is not present in buck converters where by applying current to the inductor, the same current is flowing in the load as well.
Generally this synthetic ripple signal is fed to a fast comparator that determines the charge and discharge timing of the inductor. For buck converters the implementation of a pseudo hysteretic control is relatively simple because the output stage of the buck, along with the inductor and the output capacitor, forms the integrating section of the converter that can be seen as a delta sigma converter. As mentioned above, the buck converter charges the inductor while supplying current to the load.
The intrinsic delay of the boost architecture, deriving from the fact that the boost does not supply current to the load while charging the inductor, makes the implementation of an hysteretic approach much more difficult to obtain. However as taught in the recently filed patent application (U.S. Ser. No. 12/930,498) by the same inventors, by summing a signal in phase to the inductor charge current to a signal proportional to the output voltage, an hysteretic approach can successfully be implemented for a boost power converter as well.
This method of obtaining a fast switching pseudo hysteretic boost converter enables the use of hysteretic approach in both modes of operation of a bidirectional power converter. However several challenges have to be overcome. The ideal bidirectional power converter could make use of a single feedback network that generates the synthetic ripple in both modes of operation. Although not a necessary requirement, the optimization of the control loop by combining the feedback networks would simplify the circuitry and increase the efficiency in light load conditions for effect of a reduced quiescent current.
In addition, since the main utilization of a bidirectional power converter is related to the charge and discharge of a battery, typically a battery charger is required to operate in “constant current” mode or in “constant voltage” mode depending on the voltage of the battery itself. The constant current mode is required when the voltage of the battery is quite lower than its desired final (fully charged) value. The level of charging current may be different from battery to battery and it is generally dependent on the total charge of the battery and referred to as trickle charge current or full current. In this mode of operation the switching power converter has to operate with minimum possible level of current ripple maintaining the same level of average charge current.
The hysteretic approach of constant current mode at constant switching frequency presents its challenges and the applicants are not aware of any such implementation by third parties. The constant voltage mode is generally phased in smoothly in the control by the use of transconductance operational amplifiers when the battery voltage approaches the full charge. In that case the charge current is gradually decreased and the control gradually transitions from constant current mode to constant voltage mode and it is more similar to the one commonly used for buck power converters. Here the main challenge is to operate with a pseudo hysteretic approach because the battery acts like a large output capacitor complicating significantly the stability of the system.
In fact a large output capacitor introduces a low frequency pole in the frequency response of the system. A low frequency pole inherently makes the system very slow to react to transient, but charging a battery does not require fast frequency response. However, if precaution is not taken, the low frequency pole introduced by the effective large capacitance of the battery could induce low frequency oscillation of the output voltage. Another challenge is to maintain the same switching frequency during the whole phase transition migrating from the constant current mode to the constant voltage mode.
A further challenge is characterized by the fact that when the full charge is approached, and if no load is draining the battery, the system may tend to operate in DCM (Discontinous Conduction Mode). In DCM mode the system operates in PFM (Pulse Frequency Modulation) and the charge is delivered in small pulses to the battery. For high frequency operation the inductor value is quite small and it is important to guarantee controlled current pulses in amplitude in addition to a stable system.
It is therefore a purpose of the present invention to describe a novel structure of a bidirectional switching power converter with synthetic ripple generation that can operate at high switching frequency with pseudo-hysteretic control, operating at constant frequency and with high efficiency both in CCM and DCM depending on the load conditions. It is another purpose of the present invention to describe a fully hysteretic battery charger that may be part of the bidirectional hysteretic switching power converter.